1. Field of the Invention.
The invention relates to the preparation of low molecular weight perfluoropolyethers by a process for the dissociation of high molecular weight perfluoropolyethers obtained from photochemical oxidation processes of perfluoroolefins or by anionic polymerization of their epoxides.
2. The Prior Art.
It is known how to prepare perfluoropolyethers which are stable at ambient temperature and are obtained from perfluoropolyperoxides as are described respectively for example in British patent Nos. 1,226,566 and 1,104,482.
By using perfluoropropene as a starting product it is possible to obtain compounds which, after thermal or photochemical reduction of the peroxy precursor, have general formula: ##STR1## where A is an end group of neutral nature of the type: --CF.sub.3, --C.sub.2 F.sub.5, --C.sub.3 F.sub.7 and Z is an acid group ##STR2## or a derivative thereof or a group --CO--CF.sub.3. The monomeric units with indexes m, n and r are statistically distributed along the chain, the values of indexes m, n, r vary from 0 to 200 (their sum being always higher than zero) and depend on the photo-oxidation reaction parameters.
By neutralization of the compounds of formula (I) by treatment with gaseous fluorine at temperatures ranging from 100.degree. to 200.degree. C., the corresponding neutral products are obtained wherein Z is converted into a group A' having the same meaning as A.
By using tetrafluoroethylene as a starting product there are obtained perfluoropolyethers which, after thermal or photochemical reduction of the peroxy precursor, have general formula: EQU Z'--O--(CF.sub.2 CF.sub.2 O).sub.p --(CF.sub.2 O).sub.q --Z (II)
wherein Z and Z' are acid groups ##STR3## or --CF.sub.2 COF or derivatives thereof, or --CF.sub.3,--C.sub.2 F.sub.5, monomeric units p and q are statistically distributed along the chain, the values of the indexes are such that the p/q ratio ranges from 0.5 to 2 and they depend on the photo-oxidation reaction parameters.
By neutralization with fluorine--as already indicated for the compounds of formula (I)--there are obtained corresponding neutral products in which Z' and Z, like or unlike each other, are --CF.sub.3 or --C.sub.2 F.sub.5.
Finally, there are also known perfluoropolyethers prepared by polymerization, in the presence of anionic catalysts, of the perfluoropropene epoxide, as is described e.g. in U.S. Pat. No. 3,250,808. The general formula attributable to these products is: ##STR4## wherein Y is an acyl fluoride group ##STR5## and s is an integer higher than zero.
By neutralization with fluorine there are obtained neutral compounds wherein Y is --C.sub.2 F.sub.5.
The processes for the preparation of such perfluoropolyethers lead to a distribution of the product molecular weights which tends towards the high molecular weights for which it is difficult to find practical appliances.
The preceding Italian patent applications 21051 A/84 and 21052 A/84 filed by the Applicant disclosed a process for the dissociation of high molecular weight perfluoropolyethers by dissociation in the presence of Al, Ti, V, Co, Ni fluorides or oxyfluorides, at temperatures ranging from 150.degree. C. to 380.degree. C.
Such treatments permits to obtain perfluoropolyethers having a lower and regulated molecular weight, which are the products most utilizable in practice. It is well known that the low molecular weight products are used as operative and testing fluids in electronics. The mean molecular weight products are utilized as operative fluids in the vacuum sector.